Model cystine peptides related to specific sequences in human immunoglobulins, and thioredoxin-thioredoxin reductase will be synthesized. The reactivity of the disulfide bond in these systems will be studied by disulfide interchange, electrochemical, and enzymic reduction, rates of nucleophilic cleavage, and other physical methods. The pancreatic enzyme ribonuclease A, spin-labeled at His-105, will be used as a model to evaluate the reactivity, pairing, and the specific role of cystine residues present in larger polypeptides. Conformational changes in the modified protein will be evaluated by enzymic activity, electron spin resonance spectroscopy, and optical rotatory dispersion-circular dichroism studies. Annealation experiments using synthetic peptides are also proposed.